The present invention relates generally to the field of current sensing devices. More particularly, the invention relates to a wide range current sensing arrangement suitable for use with different sizes of current transformer coils.
A range of devices are known and are currently in use for measuring current in electrical systems. Such current measurements are used for a many purposes, including for monitoring and control, both manual and automated. For example, in a single-phase AC electrical system, a current may be measured to evaluate the normal operation of electrical equipment, analyze power usage, control electrical components, or determine when an abnormal condition exists. In three-phase AC electrical systems, similar purposes may exist for measuring currents in individual phase conductors. Moreover, currents are often detected for multiple phase conductors to detect phase imbalances that could be indicative of ground faults.
A common technique for measuring currents is to pass a conductor through a ring-like coil and measure current induced in the coil resulting from the field produced by a current flowing through the conductor. Practical product offerings of this type, however, may require different sizes of current transformer coils. These sizes may be required to accommodate different sizes or numbers of conductors. For example, current through smaller conductors is more accurately measured by the use of transformer coils with a smaller opening or window through which the conductor passes. Larger conductors require larger windows. Where three phase conductors are to be routed through a single transformer coil, such as for ground fault detection, the window in the coil must be sufficiently large to accommodate the multiple conductors.
In general, the most important criterion for a current measuring system is the ability to measure current accurately. The range of primary conductor currents may be accommodated by using multiple different transformer coil sizes. Ranges of measured currents can range from sub-Ampere levels to thousands of Amperes. Although this may not be the current actually measured (e.g., in ground fault protection systems where imbalances are measured), the current through the primary conductors determines the sizes of these conductors. It is these sizes that dictate the window size for the current transformer coil. In the case of ground fault detection, the anticipated detected current levels can range from milli-Amperes to several Amperes of current, although somewhat larger coil windows may be required to accommodate the multiple conductors.
Generally, the sizes of burden resistors, as well as the number of turns of wire in the transformers must be changed when the current range or window size is changed. For example, a current transformer is modeled as a current source in parallel with a magnetizing impedance (jXLm), in parallel with the sum of a burden resistor value and the resistance of the coil winding (Rb+Rw). The number of turns must normally be increased when the diameter of the transformer coil increases due to the decrease in the magnetizing impedance. This impedance is inversely proportional to the mean diameter of the transformer coil ring. As a result, a current divider is formed with the impedance and the burden resistor value. If the value of jXLm/(jXLm+Rw+Rb) is approximately equal unity, then virtually all of the current source current will flow into the burden resistor. This will result in a minimum error for the voltage developed on the burden resistor.
However, if the number of turns or the burden resistor value changes, the net gain of this circuit also changes. This can be problematic due to the electronic circuitry utilized, which will not otherwise account for the new scaling factor resulting from the changed gain. Existing techniques for accounting for this change include varying a number of turns for different sizes of transducer coils, and changing the burden resistor value to match the value of jXLm. However, this can lead to errors in adjustment, selection, and so forth.
There is a need, therefore, for an improved technique for measuring current for a wide range of conductors and anticipated current values.